Abstract
Abstract We present a near-infrared K-band R ≃ 1500 Keck spectrum of S68N, a Class 0 protostar in the Serpens molecular cloud. The spectrum shows a very red continuum, CO absorption bands, weak or nonexistent atomic metal absorptions, and H2 emission lines. The near-IR H2 emission is consistent with excitation in shocks or by X-rays but not by UV radiation. We model the absorption component as a stellar photosphere plus circumstellar continuum emission with wavelength-dependent extinction. A Markov Chain Monte Carlo analysis shows that the most likely model parameters are consistent with a low-temperature, low-gravity photosphere with significant extinction and no more than modest continuum veiling. Its T eff ≃ 3260 K effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity log g ≃ 2.4 cm s−2 is approximately 1 dex lower. This implies that the radius of this protostar is a factor of ∼3 larger than that of 106 year old T Tauri stars. Its low veiling is consistent with a circumstellar disk having intrinsic near-IR emission that is less than or equal to that of more evolved Class I protostars. Along with the high extinction, this suggests that most of the circumstellar material is in a cold envelope, as expected for a Class 0 protostar. This is the first known detection and analysis of a Class 0 protostar absorption spectrum.
Highlights
The physical stages and observational properties of low-mass protostellar evolution have become clear over the past several decades
Along with the high extinction, this suggests that most of the circumstellar material is in a cold envelope, as expected for a Class 0 protostar
We describe a likely model for the observed absorption spectrum of Serpens S68N and describe the Markov Chain Monte Carlo (MCMC) analysis we perprovides rapid parameter estimation by swift compuformed to estimate the most likely model parameters
Summary
The physical stages and observational properties of low-mass protostellar evolution have become clear over the past several decades. ALMA observations are revealing high-resolution details of their envelopes and outflows(e.g., Evans et al 2015; Aso et al 2017). Andre et al (1993) noted that VLA 1623, the Class 0 archetype, was undetected at λ < 24 μm wavelengths at the time of that study. This was modeled as being due to the extremely high AV 1000 mag extinction of its massive (∼ 0.6 M ) circumstellar envelope. The Spitzer c2d Legacy survey detected a few dozen Class 0 YSOs (Evans et al 2009) in nearby dark clouds, and many of them have F3.6μm 1 mJy (Enoch et al 2009). Some near-to-mid-IR flux is clearly leaking out through holes in the envelopes that surround these youngest stars
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